This invention relates to a capsule or encapsulated toner to be used for developing electrostatic latent images in electrophotography or electrostatic printing.
Hitherto, a large number of electrophotographic processes have been known as disclosed in U.S. Pat. No. 2,297,691, Japanese Patent Publication Nos. 23910/1967 and 24748/1968. Generally speaking, these processes comprise steps of forming electrical latent images on a photosensitive member through utilization of a photoconductive substance according to various means, developing the latent images by use of a toner, transferring the toner images on a transfer material such as paper, if desired, and fixing the images by heating, pressure or a solvent vapor to obtain a copy. The method of securing the toner onto a material on which the toner is to be fixed by pressurization is disclosed in U.S. Pat. No. 3,269,626 and Japanese Patent Publication No. 102624/1973, and it has a number of advantages, including conservation of energy, no pollution, copying without waiting time after turning on power source for copying machine, no danger of scorching of copying paper, possibility of high speed fixing and simple fixing device.
However, in such a pressure fixing method of the prior art, no satisfactory fixing characteristic could be obtained without application of a special treatment on the image bearing member and, moreover, the fixing pressure required is so high as 200 to 300 Kg/cm.sup.2. Further, for the toner material for pressure fixing, soft materials have frequently been utilized, with the result that the toner is poor in pot life, liable to cause unfavorable phenomena such as coagulation between toner particles during storage and result in coalescence or further blocking, filming on the drum surface, carrier contamination, fixing roller-offset, etc. In recent years, in order to solve the problems as mentioned above, a large number of microcapsule toners have been proposed (see Japanese Laid-Open Patent Publication No. 139745/1975). However, there are still involved many problems in such microcapsule toners and their preparation methods.
For example, in the method in which core particles are previously formed and then encapsulated, granulation is frequently conducted with the aid of an emulsifier and a dispersant. However, due to the emulsifier and the dispersant employed, depending on the conditions, a large amount of emulsified core particles may be generated, or the particles once formed may coalesce again to form coarse particles and result in particles with a very wide particle size distribution. Further, when the phase separation method is employed in the encapsulation step, due to agglomeration of particles in a dispersing medium and dissolution of the core material into the dispersing medium in which the shell material is dissolved, when a poor solvent is added, microcapsule toner with coarse particle sizes may be obtained, or nonencapsulated particles consisting only of core particles may be by-produced. In some cases, particles consisting only of the shell material may also be by-produced. Even when the spraying method, for example, is employed in the encapsulation step, in which core particles once formed are dispersed in a solution of the shell material and the dispersion is discharged by means of a binary fluid nozzle or a disc atomizer to coat the surface of core particles with the shell material, the above problems can hardly be solved fundamentally. Thus, it has been desired to form microcapsules having a uniform particle size distribution with low energy consumption for granulation and at good efficiency. Further, in addition to particle size distribution, the capsule toners of the prior art fail to cover the surface of core particles completely on account of the interfacial free energy to result in formation of defective films or cause readily interfacial peel-off. For this reason, staining of a toner carrying member such as a sleeve and lowering in image density are frequently observed.
Generally speaking, the surface of a magnetic material is highly hydrophilic and a magnetic material is localized selectively on the surface of core particles during formation of core particles in an aqueous system, as is confirmed by observation with a scanning type electron microscope. As a consequence, when hard shell film is formed on the surface of core particles, no sufficiently high electrical resistance can be obtained, whereby the toner image obtained by developing can be transferred with very poor transfer efficiency and transfer irregularity can be caused. Accordingly, various attempts have been made. For example, surface treatment with a hydrophobicity modifying agent is previously applied on the magnetic material; an additional intermediate insulating layer is provided between the core particle and the shell film; and the shell film thickness relative to the core particle is set sufficiently greater. Although the electrical resistance has been increased to some extent by such an attempt, the improvement is not sufficient. Moreover, various problems are involved such that the production steps became very complicated, and also that the thickness of the shell material cannot be increased unlimitedly.